Method for making metal composites



M T E E P O W H C s D A METHOD FOR MAKING METAL COMPOSITES 3 Sheets-Sheet, 1

Original Filed June 25, 1959 HOT ATTORNEY Dec. 15, 1970 A. D. SCHWOPE ETAL. 3,546,769

METHOD :FOR MAKING METAL COMPOSITES 3 Sheets-Sheet (5 Original Filed June 25, 1959 40 vol FIBER 30 vol FIBER FIGA TEMPERATURE F E v P R O E J WHB mwm /l TSJ H WDW I Y 5 VRTW m N inn-.5 R G TBW O ROD a T AR T F 4 A R O I R R k IOB E E H 5 B B n m l L I. v m w m w 0 I 4 w E R I I U T I m I m mwE B P H M O E W I T P M A v 6 O o I m O M 4 R T I A I. M O W H 9 8 7 6 United States Patent Int. Cl. B22f 3/24 US. Cl. 29-4205 7 Claims ABSTRACT OF THE DISCLOSURE A composite metal structure of a powder metal matrix and a plurality of discrete metal fibers of a different metal distributed through said powder matrix. The matrix metal in powder form being combined, pressed, solid state sintered and worked to produce the composite.

This application is a division of co-pending application Ser. No. 315,564, now abandoned, and a continuation of application Ser. No. 822,838, filed June 25, 1959, by Arthur D. Schwope et al., now abandoned.

This invention relates to a method for making composite metal structures of powder metal and discrete metal fibers.

The alloying of elemental metals, metalloids, and nonmetals to achieve properties not possessed by the individual constituents is, of course, a very ancient art and is by far the most common means for achieving this result. However, the advent of or rapid development in such fields as atomic energy, rocket propulsion, guided missiles, and supersonic speed aircraft has created stringent requirements for metals having unique combinations of properties such as, for example, lightness in weight coupled with oxidation resistance and high temperature strength. In many cases it is impossible to attain the desired combinations of properties by means of conventional alloying procedures and, where possible, it is frequently economically unfeasible or otherwise impractical to do so.

In the prior art it as already been recognized that some benefit can be derived from a structure consisting either entirely of metal fibers, or in which a self-containing fiber mass is infiltrated by another metal. In such a structure the fibers are usually mechanically interlocked and not arranged in any particular direction. This concept has been utilized primarily to provide a porous body. However, the invention disclosed herein relates to a composite in which the fibers are without any structural rigidity apart from the matrix. There is no mechanical interlocking and, moreover, it is preferred that at least the majority of fibers do not touch each other to minimize stress concentration effects. The instant invention provides a structural material exhibiting improved strength over and above the strength of the matrix, for instance tensile strength and the modulus of elasticity, as hereafter further described.

It is believed that these advantages are at least in part achieved due to the bond between the fiber and the matrix, the discreteness of the fiber and the unidirectional arrangement of the fibers. It is, of course, a requisite that the fibers have the preferred properties, such as tensile strength and modulus of elasticity, which are to be imparted to the matrix.

It is a fundamental object of the present invention to overcome at least one of the disadvantages and/or limitations of the metallurgical art as outlined above.

More specifically, it is an object of the invention to provide novel, composite metal structures which exhibit highly desirable specific properties and/or combinations of properties.

A particular object of the invention is the provision of light-weight, corrosion-resistant metal structures having good high temperature strength.

A further object is the provision of improved methods for producing composite metal structures.

An aspect of the present invention resides in the method of fabricating a composite metal structure which consists of a matrix powder metal and discrete fibers composed of another metal and distributed through the matrix without forming structural rigidity in the fiber mass independent of the matrix powder metal. The matrix metal in powder from and the fibers are combined, pressed, sintered and worked to produce the composite structure in which the fibers are substantially unidirectionally oriented.

Additional objects of the invention, its advantages, scope, and the manner in which it may be practiced will be apparent to those conversant with the art from the following description and subjoined. claims taken in conjunction with the annexed drawing in which:

FIG. 1 is a perspective elevational view, partly in section, diagrammatically illustrating the method and articles contemplated by the invention; and

FIGS. 2, 3, 4 and 5 are graphic presentations of properties of metal structures according to the invention.

For the sake of example and literary ease the invention will be described in detail as applied to a titanium matrix and molybdenum fibers; however, it will be appreciated that a wide variety of metals and alloys and combinations thereof can be employed to obtain composites having particular properties or sets of properties.

The metallic structures embraced by the invention are bodies (e.g., rods, bars, plates, etc), of a matrix metal. titanium in the exemplary embodiment, containing fibers of a metal (e.g., molybdenum) possessing properties which it is desired to impart to the matrix metal.

The structures fall into two general categories hereinafter referred to as the long fiber type and the short fiber type. It is pointed out that the terms long and short are used relative to each other and allude to the length of the fibers, not in the completed structure but at the start of its fabrication.

As used herein, the term fiber" is employed and intended to embrace small diameter filaments ranging up to a maximum initial diameter of about 0.020 inch. Theoretically there is no minimum diameter for the fibers employed in short fiber composites; however, in practice, the thinnest filaments would have a diameter in the order of a few microns or less. In the long fiber composites the minimum diameter would be dictated by the need to handle and position individual fibers as will be seen as this description proceeds.

The fibers employed in structures on which data are given herein were of commercially available electroetched, mechanically straightened 0.010 inch diameter molybdenum wire.

Considering first the long fiber type structure, the physical configuration of the composite will be more readliy appreciated from the following description of an exemplary manner of its fabrication.

The fibers are axially disposed in a clean mild steel tube one end of which is then crimped shut on the wires and welded. The tube then is filled with matrix metal, in powder form. Preferably, the tube is vibrated to insure that the powder is well tamped down around the wires. After filling, the open end of the tube is crimped and welded closed on the free ends of the wires thus sealing the assembly and maintaining the wires running axially through the tube. The tube is then heated and rolled in rod rolls. Working temperatures in the range of about 1450 to 1800 F. give good results for titanium matrix with molybdenum fibers, with the lower temperatures used as the piece is progressively reduced. For different combinations of metals working temperatures suited to the particular materials would be employed but care must be taken that the working temperature will not cause the powder nor the fibers to change into a liquid or semi-liquid state.

A solid state bond is preferred since the mechanical properties of the fibers are usually detrimentally affected by contact with molten matrix materials. Placing the fibers in a liquid or semi-liquid state will cause all advantageous mechanical properties to be lost.

After reducing the diameter of the tube by about 75 to 99 percent, the jacket is stripped and the unclad rod cold rolled to improve the surface finish.

The reduction of the piece is effective upon the fiber as well as the matrix material. Consequently, the finished structure comprises a compacted matrix with a multitude of very thin fibers extending through the body in one direction and individually bonded to the matrix. Inasmuch as the long fiber composites comprise continuous, unidirectionally oriented fibers, they exhibit anisotropic properties. This must be taken into account in the design of articles to be fabricated from the material. In addition, this characteristic may be employed to improve the isotropy of matrix materials which are naturally anisotropic.

The significant improvement in tensile strength exhibited by the long fiber structure will be seen by reference to FIG. 2. It will be noted that the degree of improvement is progressively more pronounced with increasing temperature. In addition the tensile strength varies directly with the volume percentage of fiber in the structure.

It is believed that the marked improvement in the composites is due at least in part to the fact that, at a given working temperature, one of the materials, e.g., the matrix is hot Worked whereas the fiber is cold worked or vice versa.

While the improvements in properties achieved by long fiber type structures are satisfactory, the fabrication techniques are somewhat fussy with regard to the arrangement of the fibers. However, it has been discovered the short fibers, randomly oriented, provide substantially equivalent results and, because the fabrication procedures are much simpler, these are Preferred.

In a preferred method of fabricating the short fiber type structure, short fibers, e.g., in the order of 0.1 to 0.25 inch in length are admixed and blended with the matrix powder. At this stage, the fibers are oriented at random. The blended powder then is compacted and vacuum sintered. The sintered billets then are canned and extruded into rod as shown schematically in FIG. 1. In this figure, designates a conventional extrusion die, and 12, 12 a pair of rod rolls. The billet 14 entering the die is shown to contain randomly oriented fibers 16. In the extruded rod 14a leaving die 10 and prior to entering rolls 12, 12' fibers 16 have partia ly assumed a unidirectional orientation. The fibers in the finished section 14b of the rod emerging from rolls 12, 12' are substantially unidirectionally oriented. Of course in actual practice the fibers are not completely and perfectly re-oriented but the proportion and degree of unidirectional orientation is quite high and effective. It should be noted that FIG. 1 is an entirely schematic representation of one exemplary type of working technique. Actually, this working would be a two step operation. Moreover, a wide variety of working methods can be employed, e.g., forging, swaging, rolling, extrusion, direct rolling of powder, etc.

While the process parameters employed are not critical and vary with the particular materials operated upon,

following are process details for the specimens on which data are given hereinafter.

The fibers consisted of 0.010 inch diameter molybdenurn wire varying in length from 0.1 to 0.25 inch. The matrix powder was unalloyed titanium or Ti-6Al-4V alloy. The billets were compacted at 70 tons per square inch, vacuum sintered for one hour at 1800 F., canned in mild steel and extruded to /8 inch rod at 1800 F. The case was stripped (although it can be left on) and the extruded rod re-canned in a stainless steel tube, heated to 1450 F. and hot rolled to 4 inch diameter.

The cladding was then removed and the rod cold worked to inch diameter fo lowed by two hour anneal at 1350 F.

FIG. 3 demonstrates the improvement in the tensile strength of respective short fiber type structures of unalloyed titanium containing 10 to 20 volume percent molybdenum fiber as compared with a control specimen of the same material subjected to the same fabrication conditions but containing no fiber. It will be readily apparent from inspection of FIGS. 2 and 3 that the properties of the matrix material are improved by the addition of either continuous or discontinuous fibers. In comparing FIGS. 2 and 3 it should be noted that, while the matrix material in each case was unalloyed titanium,

it varied in the degree of purity; this accounts for the difference in the tensile strength of the control specimens containing no fiber.

Another important property of structural metals is the modulus of elasticity. The improvement in this parameter exhibited by composite structures comprising a Ti6Al-4V alloy matrix and, respectively, 20, 30, and 40 volume percent of discontinuous molybdenum fibers as compared with a control specimen of the same alloy as shown in FIG. 4. It will be noted that the modulus of elasticity increases directly with the volume percent of molybdenum fiber. In addition the relationship of the modulus to temperature is generally linear up to 1400 F. whereas for the specimen containing no fiber it drops off rapidly above about 600 F.

The effect of various volume percentages of molybdenum fiber (20, 30 and 40%) on the modulus of elasticity to density ratio of Ti-6Al 4V alloy over a range of temperatures is shown in FIG. 5.

From the FIG. 5 curves it will be evident that structures containing at least 20 volume percent molybdenum fiber have a notable advantage over the specimen with no fiber with respect to modulus to density ratio at temperatures above about 400 F.; with 30 volume percent fiber, the advantage exists at temperatures above F.; and with 40 percent fiber, the advantage is much greater and is manifested at all temperatures.

To obtain the fullest benefits of the molybdenum fibers at high temperatures the composites may be suitably coated or other means employed to prevent oxidation of the fiber. This is true of any fiber material which tends to oxidize readily under conditions of use.

While the fabrication techniques described hereinabove by way of example, involve powered metallurgy, it should be noted that although such techniques are preferred they are not always essential. Thus, for example, where the melting point of the matrix metal is substantially lower than that of the fiber, composite structures according to the present invention can be fabricated by melting processes.

As previously explained, the invention is applicable to a wide range of different metal matrixes and fibers which are not physically or chemically incompatible with each other or with the fabrication techniques involved. Thus, the matrix metal must be one which is susceptible of being formed by powder metallurgy or must have a substantially lower melting point than the fiber; the fiber metal must be susceptible of being formed as a thin filament, and of bonding to the matrix metal with the application of heat and pressure. Of course, the fiber metal should possess some property or properties which it is desired to impart to the matrix. In some cases it may be necessary to employ fibers of two or more different materials to achieve the desired result.

While there have been described what at present are believed to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claims all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A method of fabricating composite metal structures consisting of a matrix powder metal and discrete fibers of another metal distributed therethrough, said fibers being composed of a material capable of imparting an increase in strength to the composite over the strength of said matrix metal, wherein said matrix metal in powder form and said fibers are combined, heated to a temperature to effect solid state bonding of said fibers to said matrix and worked to produce said structure without forming structural rigidity in the fiber mass independent of the matrix power metal.

2. A method according to claim 1, wherein said fibers initially are randomly oriented and working includes the progressive elongation of said structure so as to elongate and substantially unidirectionally orient said fibers.

3. A method according to claim 1, wherein the working is carried out at a temperature which constitutes hot working for one of said metals and cold working for the other.

4. A method according to claim 1, wherein said fibers are discontinuous and discrete and uniformly distributed throughout said matrix.

5. A method of fabricating composite metal structure consisting of a matrix powder metal and filamentary particles of at least one other metal distributed therethrough, said fibers being composed of a material capable of imparting an increase in strength to the composite over the strength of said matrix, comprising: preparing a physical mixture consisting of said matrix metal in powder form and randomly oriented discontinuous discrete fibers of said other metal; forming a sintered compact of said mixture without forming structural rigidity in the fiber mass independent of the matrix powder metal; and heating to a temperature to effect solid state bonding of said fibers to said matrix and working said compact so as to density it and orient a major proportion of said fiber substantially to a single direction. parallel to each other and to the direction of working.

6. A method of fabricating a composite metal structure composed predominantly of a titanium powder matrix and discrete fibers of molybdenum distributed therethrough, comprising: preparing a mixture of titanium powder and about 3 to volume percent of molybdenum fibers having average diameters and lengths in the order of .01 and 0.1 inch, respectively; hot pressing said mixture to form a compacted billet without forming structural rigidity in the fiber mass independent of the matrix powder metal; sintering said billet at a temperature to efiect solid state bonding of said fibers to said matrix; extruding said billet into rod; and rolling said rod whereby a major proportion of initially randomly oriented fibers in said matrix are re-oriented substantially to a single direction.

7. A method of fabricating a composite metal structure consisting of a matrix metal and discontinuous discrete fibers of at least one other metal extending therethrough, said fibers being composed of a material capable of imparting an increase in strength to the composite over the strength of said matrix, comprising: arranging a plurality of said fiber extending axially through a malleable metal tube; filling said tube with said matrix metal in powder form; sealing the ends of said tube; and heating to a temperature to effect solid state bonding of said fibers to said matrix and working said tube to compact and densify its contents without forming structural rigidity in the fiber mass independent of the matrix powder metal.

U.S. Cl. X.R. 

